1. Field of the Invention
The present invention relates to a flat multicore wire for use for wiring between flexible printed circuit boards and a method of forming the same wire, and more particularly to a structure for the end portion of such a flat multicore wire and a method of forming the end portion of the same wire.
2. Statement of the Prior Art
There are known a number of types of flat multicore wires, some of which are disclosed in the official gazettes of Japanese Utility Laid-Open Nos. 196517/1988, 194413/1988 and so forth. The gist of the above official gazettes is a flat multicore wire having an end structure in which round wires are disposed in parallel in a plate-like fashion at predetermined intervals, in which coatings of the respective wires are peeled off at the end portion by a slightly short distance so as to expose conductors threat, in which pieces of connecting tape are applied over the top and bottom surfaces of the coated portion of a group of round wires along the proximal end portion of the exposed conductors (hereinafter, simply referred to as a conductor) so as to secure the parallel posture of the group of wires, thereby allowing the conductors to act as a contact portion which is brought into contact with another element. There is disclosed another type of a flat multicore wire in which a sheath body is provided at an intermediate portion of the conductors along the length thereof for holding a group of conductors altogether with a view to preventing bending of the known conductors.
Of the above two types of prior art flat multicore wires, in a flat multicore wire of the former type, since the rigidity of the conductors projecting in a cantilever-like fashion is not sufficient, the conductors are easy to "bend or buckle" due to insertion resistance generated when a connector is fitted thereover. In order to compensate for the insufficient rigidity even to a small extent, wires to be used are limited to tinted wires or solid ones in terms of construction, and thus since stranded wires are not suitable for use with a flat multicore wire of this type, the flexibility of the whole wire including the contact portion is not good. Thus, the flat multicore wire of the former type has a drawback in that it is not suitable for applications where connections are made while the wire is being bent and/or where the wire is repeatedly bent at the intermediate portions along the length thereof.
In a flat multicore wire of the latter type in which the rigidity of the conductors is intended to be improved, although the "bending or bucking" inherent in the flat multicore wire of the former type is able to be prevented, when inserted into a connector, the sheath body also has to be inserted together with the conductors, and this requires a great magnitude of insertion force, resulting in a remarkably low operating efficiency.
Means for bonding a conductor to an insulating film or the like utilized in forming a conventional tape-like wire are classified into the following three methods. First and second methods are disclosed in the official gazettes of Japanese Utility Model Publication No. 48691/1976 and Japanese Patent Laid-Open No. 10702/1980, respectively. The first method comprises the steps of "providing a thermoplastic or thermosetting bonding layer on an insulating film or the like, and fusion bonding a conductor to the insulating film or the like." The second method comprises the step of "bonding an insulating film or the like to a conductor by means of an energy ray-setting resin." A third method, which is disclosed in the U.S. Pat. No. 4,931,598, comprises the step of "bonding an insulating film or the like to a conductor by means of a cold-setting adhesive."
Among the above conventional bonding means, although the first method is common and widely propagated, in a case where a thermoplastic bonding layer is used, the layer gets plasticated depending on the temperature of the environment in which it is used, and therefore there is a risk of the portion performing the bonding function running off. Moreover, there is a drawback to applying a thermoplastic or thermosetting bonding layer on a round conductor which cannot provide a wide bonding area thereon in that sufficient bonding force cannot be obtained.
Furthermore, although the second and third methods can provide strong bonding force, it takes a long time to complete bonding, and facilities exclusively used for applying adhesive is required. Thus, these two methods also have drawbacks that costs are increased, and that productivity is decreased.
There are a number of known examples of an intermediate peeling-off method for peeling off the intermediate portion of the coating of a wire so as to expose a part of the conductor thereof, and they are classified into the following three methods. A first method comprises the steps of cutting the coating of an intermediate portion of a wire and axially separating the coatings so cut from each other so as to expose a part of the conductor therebetween. A second method, as disclosed in the official gazettes of Japanese Patent Publication No. 24441/1981, and Japanese Patent Laid-Open Nos. 58412/1986 and 8217/1991, comprises the steps of circumferentially cutting the coating of a wire at two portions that are spaced by a predetermined distance by means of a knife-type cutter, axially cutting the coating between the circumferentially cut portions, and peeling off the coating of the portion so cut. A third method comprises the step of rotating a wire brush so as to take off the coating of a predetermined portion of a wire.
The first method is common and widely used, since it is simple. However, since the separated coatings tend to slightly move back towards their original positions, it is not possible to obtain high accuracy. Moreover, since the coating gets soft when it is warm, while it gets hard when it is cold, the first method has a drawback in that the distance by which the coatings move back varies depending on the condition of the coating.
Although the second method can provide high accuracy, since the coating is mechanically cut by means of a knife-type cutter, there is a risk of excessive cutting, which results in damage of the conductor, or insufficient cutting. Furthermore, the number of processing stations for the peeling-off process is increased, which makes a processing apparatus complicated and larger, resulting in a long processing time. In addition, since tailings of the coating removed after having been cut with a knife-type cutter remain on the exposed conductor, it is troublesome to remove the tailings so remaining. On top of that, the second method has a drawback in that it cannot be used for a multicore wire in which a group of wires are disposed in parallel.
Although the third method is widly used for tape wires or the like, it is easy to damage the conductor by virtue of the rotation of the wire brush, and moreover it is troublesome to collect and remove cuttings. Furthermore, these first, second and third methods are difficult to be used for wires of small diameters and/or thin-coated wires, since the conductor is easy to be damaged or broken.
In manufacturing the above-mentioned flat multicore wires, a method is utilized that comprises the steps of primarily cutting individual wires to a predetermined length, peeling off the coating at the ends of each wire, disposing a group of wires so primarily processed in parallel in a plate-like fashion, and applying pieces of connecting tape thereover, or the steps of manually forming a group of wires that are individually cut into a parallel plate-like posture, peeling off the coating at the ends of the group of wires so formed, and applying pieces of connecting tape to finish.
In the above conventional manufacturing method of a flat multicore wire, there is no continuity in the processing process from cutting of wires to finishing, and moreover peeling off the coatings at the ends of a wire is carried out one by one, and applying pieces of connecting tape is manually carried out. This increases the processing time and manhour, resulting in low productivity. Thus, the method has a drawback in that it fails to fulfil the requirements for mass-production with high productivity.